What a Spanish attack on Gibraltar might have meant

How serious would a Spanish invasion of Gibraltar in 1940 have been for Britain?

Gibraltar was heavily fortified, so Spain would have faced a difficult siege rather than a quick conquest and would have assumed grave strategic risks.

Gibraltar's fortifications would have turned a Spanish attack in 1940 into a siege rather than a straightforward invasion. The operation is judged likely to have gone badly for Spain. The strategic danger would have grown further after the United States entered the war. Spain would then have faced a much stronger Allied coalition while still contesting a heavily fortified position.